A=[10 -7 1 4 6;-8 4 -6 -10 -3;-7 11 -5 -1 -8; 3 -1 10 12 12];
A(2,:)=A(2,:)+8/10*A(1,:);
A(3,:)=A(3,:)+7/10*A(1,:);
A(4,:)=A(4,:)-3/10*A(1,:);
A(3,:)=A(3,:)-A(3,2)/A(2,2)*A(2,:);
A(4,:)=A(4,:)-A(4,2)/A(2,2)*A(2,:);
A(4,:)=A(4,:)-A(4,3)/A(3,3)*A(3,:);
A